Primary frequency standards are atomic clocks that need to operate without recalibration for long periods of time, such as several months. Recent emphasis on reducing the size, weight, and power of these devices has motivated development of miniature primary frequency standards which are based on the microwave hyperfine transition in alkali atoms, typically either Cesium or Rubidium. To achieve the required stability, one conventional approach uses laser cooled atoms that enable narrow clock linewidths in a small size. Atoms are pre-cooled for a background atomic vapor into a magneto-optical trap (MOT) or optical molasses. Atoms in the MOT serve as the source for the clock measurement. The atoms are released from the MOT and, ideally, are free from any external field (optical or magnetic). During this time, microwave spectroscopy using an external crystal oscillator is performed to measure the atomic hyperfine transition. In miniature primary standards the spectroscopy duration is typically tens of milliseconds. After the microwave spectroscopy, an optical lightwave is used to probe the populations in the individual hyperfine states, completing the spectroscopy. Information from the spectroscopic measurement is used to discipline the long-term stability of the external crystal oscillator.
These systems use an external laser that performs both the laser cooling and the probing of the atomic states after the microwave spectroscopy. The laser frequency must be precisely stabilized to the appropriate atomic hyperfine transitions. Typically, this stabilization is performed using laser spectroscopy on a separate atomic vapor cell (reference cell). Several techniques can be used, including polarization based spectroscopy and saturated absorption spectroscopy (SAS). While polarization based spectroscopy systems typically have large capture regions, these techniques require frequency calibration and can drift with changes in temperature. The SAS is self-calibrating and immune to drifts in temperature, but has a much smaller capture region, typically limited to the hyperfine linewidth.
When the laser is used to load the MOT, the light must be detuned from the atomic resonance to account for the atoms' Doppler shift. However, when the laser is used to probe the atoms after spectroscopy, it is preferred that the laser be stabilized on resonance with the atoms. The probing step typically occurs very quickly after the spectroscopy (less than 1 ms later).
The presence of external fields during atomic clock spectroscopy can provide large clock shifts, which will appear as clock biases or drift. Through the choice of appropriate clock transitions and the use of magnetic shielding, the magnetic shifts can be managed, leaving the dominant clock shift caused by atoms interacting with the near resonance light via the AC Stark shift. The Stark shift can be reduced by two ways: reducing the amount of light on the atoms during the spectroscopy, or changing the frequency of the light so that it is far off resonance with the atoms. To achieve primary frequency standard performance, the required attenuation is often larger than 80 dB even when combined with a greater than 10 GHz laser shift.
To reach these levels of extinction, larger laboratory-scale systems can use mechanical shutters, which completely block the optical lightwaves, but these are large, relatively slow, and not mechanically robust enough for continuous, long-term operation. Larger systems can also make use of acousto-optic beam shifters which provide 80 dB of attenuation and nanosecond switching speeds, but have large footprints, require Watts of optical power, and result in insertion losses as large as 3 dB.
Smaller systems striving to minimize power consumption and footprint, have tight requirements on size, speed, and insertion loss, but still have the same extinction requirements. Smaller systems have explored alternative shutter technology such as MEMs mirrors, electro-optic, or liquid crystal. While these shutters are an alternative to the mechanical shutters for the most part, these technologies fail to meet the speed and/or extinction requirement. The extinction can be increased by concatenating stages, but at the expense of increasing the insertion loss and increasing the optical footprint. Alternatively, the large laser frequency shift can be used in conjunction with these shutter technologies, but the frequency jump must be very large. Increasing the laser frequency shift during spectroscopy adds risk of re-acquiring the resonant light needed for the atomic probing, especially using SAS with the limited capture region.